The present invention relates to an optical signal detection and processing apparatus. More particularly, a three dimensional image receiver having contiguous detectors coupled to a peristaltic charge coupled device (PCCD) memory is disclosed.
Three dimensional imagers may be used in many applications, including threat warning receivers, and optical ranging and terrain mapping systems. Generally, three dimensional imaging is accomplished by taking successive samples of an object or target with an XY array. Successive samples taken rapidly, in the order of several nanoseconds apart, provide range resolution. For example, an object illuminated by a laser pulse reflects a spacial image to a three dimensional receiver. The reflections will be spread in time depending upon the depth dimension of the object. Thus, the spacial distribution of the XY images, taken rapidly as aforesaid, provide data for forming the three dimensional image.
The architecture of the detector of the present invention is primarily dictated by the requirement to compress the bandwidth of the device for improved performance. The operating frequency of the sensor of the present invention is about 50 MHz. Because the signals are extremely small, special receivers are required. Such receivers employ bandwidth compression to improve range performance. For example, a 3-D image may be taken every 30 mSec. For the range over which the receiver is operated, all of the 3-D image information arrives in less than 100 mSec. Thus, only a small part of the time between the transmit and receive pulse is used. The low operating duty cycle of the detector can be used to reduce the output bandwidth if the bandwidth compression is employed. If the system operates entirely in the charge domain, the full advantages of bandwidth compression will be realized. Because there is no interim signal amplification before read-out, wherein noise from the interim amplifies would be added, the noise bandwidth is determined by the read out rate. Operating in the charge domain thus reduces noise through reduction of the noise bandwidth and eases the speed requirement on any postprocessing.
In the United States Patent application entitled THREE DIMENSIONAL OPTICAL RECEIVER HAVING PROGRAMMABLE GAIN SENSOR STAGES executed Dec. 20, 1983, and assigned to the assignee herein, a hybrid or two tier, multilevel, staircase structure having programmable gain control and a charge coupled memory device is disclosed. The two tier structure provides the needed access between the charged coupled device memory (CCD) and a plurality of contiguous detectors. The contiguous arrangement enhances detection through the elimination of dead areas in the sensor.
In the above identified application, each detector is coupled to a programmable gain control element, which adjusts the sensitivity of the detector as a function of the anticipated light energy to be received. For example, for near distances, the signal received by the detector may be as high as 10.sup.9 photons, a level well beyond the holding capacity of a CCD. At far ranges, the detector may receive less than 10.sup.3 photons. Thus, the programmable element adjusts the gain or injection efficiency between the detector and the CCD so that the input signal to the CCD is within the charge capacity of the CCD.
The CCD is used as an analog buffer memory for fast transient recording of the signals from the detectors. A multiplexer is used to slowly read out the contents of the CCD buffer memory line by line through a read out device.
In the aforementioned patent application, the optical receiver is a hybrid or constructed in a two tiers which satisfies the requirement for continguous detectors while concurrently providing physical access to the memory. In the present invention, the detectors are made contiguous by other means. The entire optical receiver may thus be built from a single monolithic integrated circuit. The device is thereby simplified and fabrication and operation is rendered more efficient.
In the present invention, the detector is made contiguous on one side and effectively non-contiguous on an opposite side. The structure thereby provides sufficient room between the detectors on the non-contiguous side for access to the memory without the necessity of a hybrid structure.
Another aspect of the hybrid structure is that the programmable gain control element is located at the output side of the detector and is geometrically limited because gain is controlled by a programmable area division. That is, the gain is adjusted as a fraction of the physical area available for the charge entering the memory relative the total area available for the charge exiting from the detector. In the present invention, the gain control element is located at the radiation receiving side of the detector, and the gain is varied exponentially by controlling the bias over time. Thus, the gain control provided has a much greater dynamic range.
The present invention thus addresses two major problems associated with known image detectors. First, the invention provides for a monolithic contiguous image detecting structure, thereby eliminating the more complex hybrid structures. Second, programmable gain control is provided which is geometrically independent and exponentially variable over the dynamic range of the incoming radiation, thereby providing finer sensitivity, resolution and greater dynamic range.